How to recover after criss-cross merge?

[Matt Mackall]

On Wed, 2016-01-13 at 22:06 +0100, Manuel Jacob wrote:
> Hi,
> 
> In the PyPy repository [1] we have three branches (which are interesting 
> for this post): "default", "py3k", "py3.3".  "py3k" branched from 
> "default" and we regularly merge "default" into "py3k".  "py3.3" 
> branched from "py3k" and we regularly merge "py3k" into "py3.3".
> 
> Everything worked fine until someone merged directly from "default" to 
> "py3.3" (changeset 4fa19970ddeb), skipping "py3k".  When merging from 
> "py3k" into "py3.3" the next time (changeset 88aafcb7c318) we ran into 
> problems.  It showed many "ambiguous merge - picked m action" warnings.

That's fine.

> Also, some files (e.g. lib-python/3/_abcoll.py) which were removed in 
> the "py3.3" branch (but still existent in "py3k") re-appeared without 
> warning.

But that's more of a problem. Bid merge currently doesn't attempt to resolve the
ambiguity between deleted in one merge and modified in another.

> I'd prefer to only redo all changes from just before the merge from
> "py3k" into "py3.3".  I tested (locally) redoing this merge (changeset 
> 88aafcb7c318) with aeafe30eac60 as a merge base (by passing "--config 
> merge.preferancestor=aeafe30eac60").  This changeset aeafe30eac60 is the 
> last commit from "py3k" which got merged into "py3.3" before 
> 88aafcb7c318.  It seems to do exactly what I want.  However the next 
> time I merged "py3k" into "py3.3" (after in turn merging "default" into 
> "py3k") I ran into problems again (ambiguous merge warnings and 
> re-appearing files).  Is there a way to permanently fix the problem?

You can declare "everyone out of the pool", clean things up, do a proper set of
merges, and then restart work. So long as there are more recent common ancestors
to choose for future "correct" merges and no one tries to merge on their pre-
event branches, it should work.

-- 
Mathematics is the supreme nostalgia of our time.